Microwave cooking provides a rapid and efficient means of processing foodstuff, but generally does not result in surface browning of the food. In order to provide surface browning, some form of direct surface heating is required, such as by conduction through contact or by radiation.
Conventional means for providing browning with microwave energy typically comprises the use of a microwaveheatable apparatus in the microwave chamber which acts as a supplemental heating source to brown the food by radiation or conduction. Such an apparatus may become a supplemental heating source through the incorporation therewith of electroconductive members or portions which are heated by internal currents generated by the microwave energy.
A preferred form of microwave browning apparatus is a browning vessel such as a plate, platter, dish or pan composed of a glass, glass-ceramic or ceramic material to which an electroconductive coating or film has been applied, such as a tin oxide based coating. The film typically has an electrical resistance value in the range of about 90-350 ohms per square which renders it efficiently heatable in a microwave field. Upon exposure to microwave energy, the film becomes heated, and then through heat transfer the vessel is also heated to a degree sufficient to brown the surface of foodstuff contained therein in contact with the vessel. Although tin oxide coated glass microwave browning dishes have been available since the mid 1950's, U.S. Pat. No. 3,783,220 describes a presently known type of microwave browing vessel comprising a glass, ceramic or crystallized glass (a glass ceramic such as may be described in U.S. Pat. No. 2,920,971) vessel having a thin electroconductive coating of a tin oxide type upon its exterior surface. Tin oxide films, such as described in U.S. Pat. No. 2,564,706 to Mochel, consisting predominantly of tin oxide but also containing about 0.001-13% Sb.sub.2 O.sub.3 by weight, are also suitable for such application.
Microwave browning vessels of the kind described in the beforementioned patent have become available commercially, but have not been completely satisfactory due to the fact that the thermal profile developed by the coated vessels produces a relatively cold central area within the bottom of the vessel having a temperature insufficient to effect browning of the foodstuff contained therein. That is, presently available coated vessels or dishes for browning foodstuffs in microwave ovens have relatively large temperature gradients across the bottom surface thereof, with low temperatures being exhibited in central areas, and high temperatures in outer peripheral areas. As a result, the bottom central areas of such dishes may be at temperatures which are insufficient to produce browning, whereas the outer peripheral areas are at elevated temperatures tending to over brown or burn the foodstuff.
The problem of a changing temperature distribution or temperature differential (.DELTA.T) over the bottom of known browning dishes may be exemplified by the fact that, for a given preheat time of about 6 minutes, it was only possible to obtain a maximum of about 40 square inches of acceptable browning area, whereas with the present invention upwards of 65 square inches of acceptable browning area are obtainable for the same preheat time. Although not concerned with the problem of temperature distribution across the bottom of a browning dish, U.S. Pat. No. 3,539,751 was concerned with the loss of heat energy from such a dish, and attempted to overcome such loss through the use of a separate insulating implement which reflected the heat back to foodstuffs within the dish.
The present invention overcomes the problems encountered in the prior art relative to uneven browning of foodstuff within coated microwave browning dishes for use in microwave ovens, by providing a novel recess in the bottom of the dish beneath the coating, wherein the recess is downwardly open and has bounding sidewalls which communicate at their upper extent with the coated bottom surface so as to focus and redirect the microwave energy applied to the chamber of a microwave oven across the coated surface of a dish positioned therewithin and produce a substantially uniform temperature profile across the bottom of such dish thus providing uniform browning of foodstuffs contained therein.